quantum computerA computer that exploits the quantum mechanical properties of superposition in order to allow a single operation to act on a large number of pieces of data. In a quantum computer, the data to be manipulated, represented in quantum bits, exists in all possible states simultaneously, in superposition. This allows a single operation to operate over all of these states at once, in contrast with a classical computer, which must carry out an operation for each state separately. Because of the difficulty of creating environments small enough for quantum effects to emerge but sufficiently isolated to prevent interaction with outside influences such as heat, only extremely rudimentary quantum computers currently exist, though algorithms for possible future devices are being developed.

computer A type of computer which uses the ability of quantum systems, such as a collection of atoms, to be in many different states at once. In theory, such superpositions allow the computer to perform many different computations simultaneously. This capability is combined with interference among the states to produce answers to some problems, such as factoring integers, much more rapidly than is possible with conventional computers. In practice, such machines have not yet been built due to their extreme sensitivity to noise. Oxford University (http://eve.physics.ox.ac.uk/QChome.html), Stanford University (http://feynman.stanford.edu/qcomp/). A quantum search algorithm (ftp://parcftp.xerox.com/pub/dynamics/quantum.html) for constraint satisfaction problems exhibits the phase transition for NP-complete problems. (1997-02-11)